The review was done by the intelligence community's inspector general. It examined whether there were any missed opportunities to share information that could have prevented two ethnically Chechen brothers who immigrated to the U.S. from carrying out the attack.

The Russians asked the U.S. in 2011 to look into one of the brothers out of concern that he might be a threat to Russia. But they withheld some details about the potential threat until after the attack.

Tamerlan Tsarnaev (TAM'-ehr-luhn tsahr-NEYE'-ehv) died in a police chase after the attack. His brother, Dzhokhar (joh-HAHR'), has pleaded not guilty to 30 federal charges, including using a weapon of mass destruction.